1. Field of the Invention
The present invention relates to an antireflective film for an optical element adapted for use in a spectacle lens, a camera, optical communication, optical information processing, a binocular, a video still camera, a magnetooptical recording disk, a television or the like.
2. Related Background Art
In the field of antireflective film for optical elements, a MgF.sub.2 film having a low refractive index and a low absorption in the visible spectral region is widely employed. The MgF.sub.2 film was originally commercialized as an antireflective film formed by vacuum evaporation, for optical elements of glass, and is still used at the present. However, such MgF.sub.2 film formed by vacuum evaporation cannot satisfy certain optical and mechanical requirements unless it is heated at a high temperature (200.degree.-400.degree. C.) during and after the evaporation. Consequently, in the preparation of a precision optical element, such MgF.sub.2 film is extremely difficult to prepare, because of thermal deformation or similar reasons. For this reason, a MgF.sub.2 film having sufficient performance for precision optical elements and optical elements made of plastics is not available. Also since it is extremely difficult to reduce the spectral reflectance in a single-layered antireflective MgF.sub.2 film, there is ordinarily employed a multi-layered structure having three or more layers.
There has also been proposed an antireflective film principally composed of a complex oxide of Zr and Si for use on a lens, as disclosed in the Japanese Patent Laid-open Application Nos. 3-173638 and 3-162943.
Such conventional technologies require a multi-layered antireflective film, having three or more layers in order, to reduce the reflectance in the spectral reflective characteristics. Such reflective film, if prepared by sputtering or the like, has a low productivity, leading to a high production cost.